In oil and gas well drilling operations it is necessary to cement various tubular members to a subterranean formation at different points during the well drilling and completion operations. This practice is well known for various purposes, such as anchoring a surface casing to the earth to provide a solid leak-free top section of the well, and, in the lower portions of the well, to provide isolation between different subterranean zones.
Many wells are now drilled in deviated or non-vertical directions. This practice often utilizes a mud motor to rotate the drill bit without the need to rotate the entirety of the drill string. Conventional mud motors are run on a work string and are retrieved from the wellbore before the string of tubulars, typically casing, is run in the hole.
Applicant is aware that a third party has developed a mud motor that is relatively inexpensive and can be abandoned in the wellbore. This disposable mud motor is run on the end of the casing string.
During cementing operations, it is desired that the cement slurry not be pumped through the mud motor so as to prevent the mud motor from continuing to rotate. Further, mud motors have a high pressure differential across motor which may adversely affect the rate at which the cement is pumped and delivered to the annulus between the casing and the wellbore.
In order to facilitate cementing around, rather than through, a mud motor, the cement must be able to pass from a bore through the casing string to the exterior of the casing string and then be able to pass around the exterior of the mud motor. To accomplish this, ports are provided in a wall of the casing to allow cement to pass therethrough. As will be appreciated by one of skill in the art, a hole drilled through the wall is insufficient. There are many steps in the drilling process where having ports open between the interior and exterior of the casing would be undesirable. It is known that the timing of opening of ports in the casing must be controllable.
Prior art solutions have used conventional burst disks to control the opening of the ports using a predetermined pressure. Once the burst disks, positioned above the mud motor have ruptured, cement flowing down the bore of the casing exits the casing wall through the open ports created thereby for flowing the cement around, rather than through, the mud motor.
Applicant has found however, that conventional burst disks do not open reliably. Further, where a plurality of burst disks are used, if a first burst disk or a relatively small number of the plurality of disks burst, the pressure in the casing bore is relieved as the fluid flows to the wellbore, and thereafter, the pressure does not meet the threshold required to burst the remainder of the burst disks. One solution has been to attempt to significantly increase the pumping rate such that the resulting pressure is adequate to result in rupture of more of the burst disks.
Cementing operations typically require a relatively high pumping rate to ensure cement is pumped downhole through the casing bore and returned toward surface through the annulus between the casing and the wellbore. With only a single port or a small number of ports open through the ruptured burst disk or disks, the flow rate of cement is restricted to that possible through a openings or ports created by the rupture of the single burst disk or small number of disks.
Clearly there is a need in the industry for apparatus that reliably opens to permit pumping of cement through the work string, at a relatively high pumping rate, so as to flow around the mud motor and into the annulus between the casing and the wellbore.